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Major redesign of PEP 501 interpolation

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Nick Coghlan 2015-08-22 19:57:17 +10:00
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@ -1,5 +1,5 @@
PEP: 501
Title: Translation ready string interpolation
Title: General purpose string interpolation
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>
@ -18,53 +18,83 @@ transparent to the compiler, allow name references from the interpolation
operation full access to containing namespaces (as with any other expression),
rather than being limited to explicitly name references.
This PEP agrees with the basic motivation of PEP 498, but proposes to focus
both the syntax and the implementation on the il8n use case, drawing on the
previous proposals in PEP 292 (which added string.Template) and its predecessor
PEP 215 (which proposed syntactic support, rather than a runtime string
manipulation based approach). The text of this PEP currently assumes that the
reader is familiar with these three previous related proposals.
However, it only offers this capability for string formatting, making it likely
we will see code like the following::
The interpolation syntax proposed for this PEP is that of PEP 292, but expanded
to allow arbitrary expressions and format specifiers when using the ``${ref}``
interpolation syntax. The suggested new string prefix is "i" rather than "f",
with the intended mnemonics being either "interpolated string" or
"il8n string"::
os.system(f"echo {user_message}")
This kind of code is superficially elegant, but poses a significant problem
if the interpolated value ``user_message`` is in fact provided by a user: it's
an opening for a form of code injection attack, where the supplied user data
has not been properly escaped before being passed to the ``os.system`` call.
To address that problem (and a number of other concerns), this PEP proposes an
alternative approach to compiler supported interpolation, based on a new
``__interpolate__`` magic method, and using a substitution syntax inspired by
that used in ``string.Template`` and ES6 JavaScript, rather than adding a 4th
substitution variable syntax to Python.
Proposal
========
This PEP proposes that the new syntax::
value = !interpolator "Substitute $names and ${expressions} at runtime"
be interpreted as::
_raw_template = "Substitute $names and ${expressions} at runtime"
_parsed_fields = (
("Substitute ", 0, "names", "", ""),
(" and ", 1, "expressions", "", ""),
(" at runtime", None, None, None, None),
)
_field_values = (names, expressions)
value = interpolator.__interpolate__(_raw_template,
_parsed_fields,
_field_values)
Whitespace would be permitted between the interpolator name and the opening
quote, but not required in most cases.
The ``str`` builtin type would gain an ``__interpolate__`` implementation that
supported the following ``str.format`` based semantics::
>>> import datetime
>>> name = 'Jane'
>>> age = 50
>>> anniversary = datetime.date(1991, 10, 12)
>>> i'My name is $name, my age next year is ${age+1}, my anniversary is ${anniversary:%A, %B %d, %Y}.'
>>> !str'My name is $name, my age next year is ${age+1}, my anniversary is ${anniversary:%A, %B %d, %Y}.'
'My name is Jane, my age next year is 51, my anniversary is Saturday, October 12, 1991.'
>>> i'She said her name is ${name!r}.'
>>> !str'She said her name is ${name!r}.'
"She said her name is 'Jane'."
This PEP also proposes the introduction of three new builtin functions,
``__interpolate__``, ``__interpolateb__`` and ``__interpolateu__``, which
implement key aspects of the interpolation process, and may be overridden in
accordance with the usual mechanisms for shadowing builtin functions.
The interpolation prefix could be used with single-quoted, double-quoted and
triple quoted strings. It may also be used with raw strings, but in that case
whitespace would be required between the interpolator name and the trailing
string.
This PEP does not propose to remove or deprecate any of the existing
string formatting mechanisms, as those will remain valuable when formatting
strings that are not present directly in the source code of the application.
The key aim of this PEP that isn't inherited from PEP 498 is to help ensure
that future Python applications are written in a "translation ready" way, where
many interface strings that may need to be translated to allow an application
to be used in multiple languages are flagged as a natural consequence of the
development process, even though they won't be translated by default.
Rationale
=========
PEP 498 makes interpolating values into strings with full access to Python's
lexical namespace semantics simpler, but it does so at the cost of introducing
yet another string interpolation syntax.
yet another string interpolation syntax, and also creates a situation where
interpolating values into sensitive targets like SQL queries, shell commands
and HTML templates will enjoy a much cleaner syntax when handled without
regard for code injection attacks than when they are handled correctly.
This PEP proposes to handle the latter issue by always specifying an explicit
interpolator for interpolation operations, and the former by adopting the
``string.Template`` substitution syntax defined in PEP 292.
The interpolation syntax devised for PEP 292 is deliberately simple so that the
template strings can be extracted into an il8n message catalog, and passed to
template strings can be extracted into an i18n message catalog, and passed to
translators who may not themselves be developers. For these use cases, it is
important that the interpolation syntax be as simple as possible, as the
translators are responsible for preserving the substition markers, even as
@ -77,31 +107,35 @@ introduced for general purpose string formatting in PEP 3101, so this PEP adds
that flexibility to the ``${ref}`` construct in PEP 292, and allows translation
tools the option of rejecting usage of that more advanced syntax at runtime,
rather than categorically rejecting it at compile time. The proposed permitted
expressions inside ``${ref}`` are exactly as defined in PEP 498.
expressions, conversion specifiers, and format specifiers inside ``${ref}`` are
exactly as defined in PEP 498.
The specific proposal in this PEP is also deliberately close in both syntax
and semantics to the general purpose interpolation syntax introduced to
JavaScript in ES6, as we can reasonably expect a great many Python to be
regularly switching back and forth between user interface code written in
JavaScript and core application code written in Python.
Specification
=============
In source code, i-strings are string literals that are prefixed by the
letter 'i'. The string will be parsed into its components at compile time,
which will then be passed to the new ``__interpolate__`` builtin at runtime.
In source code, interpolation expressions are introduced by the new character
``!``. This is a new kind of expression, consisting of::
The 'i' prefix may be combined with 'b', where the 'i' must appear first, in
which case ``__interpolateb__`` will be called rather than ``__interpolate__``.
Similarly, 'i' may also be combined with 'u' to call ``__interpolateu__``
rather than ``__interpolate__``.
!DOTTED_NAME TEMPLATE_STRING
The 'i' prefix may also be combined with 'r', with or without 'b' or 'u', to
produce raw i-strings. This disables backslash escape sequences in the string
literal as usual, but has no effect on the runtime interpolation behaviour.
Similar to ``yield`` expressions, this construct can be used without
parentheses as a standalone expression statement, as the sole expression on the
right hand side of an assignment or return statement, and as the sole argument
to a function. In other situations, it requires containing parentheses to avoid
ambiguity.
In all cases, the only permitted location for the 'i' prefix is before all other
prefix characters - it indicates a runtime operation, which is largely
independent of the compile time prefixes (aside from calling different
interpolation functions when combined with 'b' or 'u').
The template string must be a Unicode string (byte strings are not permitted),
and string literal concatenation operates as normal within the template string
component of the expression.
i-strings are parsed into literals and expressions. Expressions
The template string is parsed into literals and expressions. Expressions
appear as either identifiers prefixed with a single "$" character, or
surrounded be a leading '${' and a trailing '}. The parts of the format string
that are not expressions are separated out as string literals.
@ -110,63 +144,68 @@ While parsing the string, any doubled ``$$`` is replaced with a single ``$``
and is considered part of the literal text, rather than as introducing an
expression.
These components are then organised into 3 parallel tuples:
These components are then organised into a tuple of tuples, and passed to the
``__interpolate__`` method of the interpolator identified by the given
name::
* parsed format string fields
* expression text
* expression values
DOTTED_NAME.__interpolate__(TEMPLATE_STRING,
<parsed_fields>,
<field_values>)
And then passed to the ``__interpolate__`` builtin at runtime::
The template string field tuple is inspired by the interface of
``string.Formatter.parse``, and consists of a series of 5-tuples each
containing:
__interpolate__(fields, expressions, values)
* a leading string literal (may be the empty string)
* the substitution field position (zero-based enumeration)
* the substitution expression text
* the substitution conversion specifier (as defined by str.format)
* the substitution format specifier (as defined by str.format)
The format string field tuple is inspired by the interface of
``string.Formatter.parse``, and consists of a series of 4-tuples each containing
a leading literal, together with a trailing field number, format specifier,
and conversion specifier. If a given substition field has no leading literal
section, format specifier or conversion specifier, then the corresponding
elements in the tuple are the empty string. If the final part of the string
has no trailing substitution field, then the field number, format specifier
If a given substition field has no leading literal section, format specifier
or conversion specifier, then the corresponding elements in the tuple are the
empty string. If the final part of the string has no trailing substitution
field, then the field number, format specifier
and conversion specifier will all be ``None``.
The expression text is simply the text of each interpolated expression, as it
appeared in the original string, but without the leading and/or surrounding
expression markers.
The expression values are the result of evaluating the interpolated expressions
in the exact runtime context where the i-string appears in the source code.
The substitution field values tuple is created by evaluating the interpolated
expressions in the exact runtime context where the interpolation expression
appears in the source code.
For the following example i-string::
For the following example interpolation expression::
i'abc${expr1:spec1}${expr2!r:spec2}def${expr3:!s}ghi $ident $$jkl'``,
!str 'abc${expr1:spec1}${expr2!r:spec2}def${expr3:!s}ghi $ident $$jkl'
the fields tuple would be::
the parsed fields tuple would be::
(
('abc', 0, 'spec1', ''),
('', 1, 'spec2' 'r'),
(def', 2, '', 's'),
('ghi', 3, '', ''),
('$jkl', None, None, None)
('abc', 0, 'expr1', '', 'spec1'),
('', 1, 'expr2', 'r', 'spec2'),
(def', 2, 'expr3', 's', ''),
('ghi', 3, 'ident', '', ''),
('$jkl', None, None, None, None)
)
For the same example, the expression text and value tuples would be::
While the field values tupe would be::
('expr1', 'expr2', 'expr3', 'ident') # Expression text
(expr1, expr2, expr2, ident) # Expression values
(expr1, expr2, expr3, ident)
The fields and expression text tuples can be constant folded at compile time,
while the expression values tuple will always need to be constructed at runtime.
The parsed fields tuple can be constant folded at compile time, while the
expression values tuple will always need to be constructed at runtime.
The default ``__interpolate__`` implementation would have the following
The ``str.__interpolate__`` implementation would have the following
semantics, with field processing being defined in terms of the ``format``
builtin and ``str.format`` conversion specifiers::
_converter = string.Formatter().convert_field
def __interpolate__(fields, expressions, values):
def __interpolate__(raw_template, fields, values):
template_parts = []
for leading_text, field_num, format_spec, conversion in fields:
for leading_text, field_num, expr, conversion, format_spec in fields:
template_parts.append(leading_text)
if field_num is not None:
value = values[field_num]
@ -176,167 +215,162 @@ builtin and ``str.format`` conversion specifiers::
template_parts.append(field_str)
return "".join(template_parts)
The default ``__interpolateu__`` implementation would be the
``__interpolate__`` builtin.
Writing custom interpolators
----------------------------
The default ``__interpolateb__`` implementation would be defined in terms of
the binary mod-formatting reintroduced in PEP 461::
To simplify the process of writing custom interpolators, it is proposed to add
a new builtin decorator, ``interpolator``, which would be defined as::
def __interpolateb__(fields, expressions, values):
template_parts = []
for leading_data, field_num, format_spec, conversion in fields:
template_parts.append(leading_data)
if field_num is not None:
if conversion:
raise ValueError("Conversion specifiers not supported "
"in default binary interpolation")
value = values[field_num]
field_data = ("%" + format_spec) % (value,)
template_parts.append(field_data)
return b"".join(template_parts)
def interpolator(f):
f.__interpolate__ = f.__call__
return f
This definition permits examples like the following::
This allows new interpolators to be written as::
>>> data = 10
>>> ib'$data'
b'10'
>>> b'${data:%4x}'
b' a'
>>> b'${data:#4x}'
b' 0xa'
>>> b'${data:04X}'
b'000A'
@interpolator
def my_custom_interpolator(raw_template, parsed_fields, field_values):
...
Expression evaluation
---------------------
The expressions that are extracted from the string are evaluated in
the context where the i-string appeared. This means the expression has
full access to local, nonlocal and global variables. Any valid Python
expression can be used inside ``${}``, including function and method calls.
References without the surrounding braces are limited to looking up single
identifiers.
The subexpressions that are extracted from the interpolation expression are
evaluated in the context where the interpolation expression appears. This means
the expression has full access to local, nonlocal and global variables. Any
valid Python expression can be used inside ``${}``, including function and
method calls. References without the surrounding braces are limited to looking
up single identifiers.
Because the i-strings are evaluated where the string appears in the
source code, there is no additional expressiveness available with
i-strings. There are also no additional security concerns: you could
have also just written the same expression, not inside of an
i-string::
Because the substitution expressions are evaluated where the string appears in
the source code, there are no additional security concerns related to the
contents of the expression itself, as you could have also just written the
same expression and used runtime field parsing::
>>> bar=10
>>> def foo(data):
... return data + 20
...
>>> i'input=$bar, output=${foo(bar)}'
>>> !str 'input=$bar, output=${foo(bar)}'
'input=10, output=30'
Is equivalent to::
Is essentially equivalent to::
>>> 'input={}, output={}'.format(bar, foo(bar))
'input=10, output=30'
Format specifiers
-----------------
Handling code injection attacks
-------------------------------
Format specifiers are not interpreted by the i-string parser - that is
handling at runtime by the called interpolation function.
The proposed interpolation expressions make it potentially attractive to write
code like the following::
Concatenating strings
---------------------
myquery = !str "SELECT $column FROM $table;"
mycommand = !str "cat $filename"
mypage = !str "<html><body>$content</body></html>"
As i-strings are shorthand for a runtime builtin function call, implicit
concatenation is a syntax error (similar to attempting implicit concatenation
between bytes and str literals)::
These all represent potential vectors for code injection attacks, if any of the
variables being interpolated happen to come from an untrusted source. The
specific proposal in this PEP is designed to make it straightforward to write
use case specific interpolators that take care of quoting interpolated values
appropriately for the relevant security context::
>>> i"interpolated" "not interpolated"
File "<stdin>", line 1
SyntaxError: cannot mix interpolation call with plain literal
myquery = !sql "SELECT $column FROM $table;"
mycommand = !sh "cat $filename"
mypage = !html "<html><body>$content</body></html>"
This PEP does not cover adding such interpolators to the standard library,
but instead ensures they can be readily provided by third party libraries.
(Although it's tempting to propose adding __interpolate__ implementations to
``subprocess.call``, ``subprocess.check_call`` and ``subprocess.check_output``)
Format and conversion specifiers
--------------------------------
Aside from separating them out from the substitution expression, format and
conversion specifiers are otherwise treated as opaque strings by the
interpolation template parser - assigning semantics to those (or, alternatively,
prohibiting their use) is handled at runtime by the specified interpolator.
Error handling
--------------
Either compile time or run time errors can occur when processing
i-strings. Compile time errors are limited to those errors that can be
detected when parsing an i-string into its component tuples. These errors all
raise SyntaxError.
Either compile time or run time errors can occur when processing interpolation
expressions. Compile time errors are limited to those errors that can be
detected when parsing a template string into its component tuples. These
errors all raise SyntaxError.
Unmatched braces::
>>> i'x=${x'
>>> !str 'x=${x'
File "<stdin>", line 1
SyntaxError: missing '}' in interpolation expression
Invalid expressions::
>>> i'x=${!x}'
>>> !str 'x=${!x}'
File "<fstring>", line 1
!x
^
SyntaxError: invalid syntax
Run time errors occur when evaluating the expressions inside an
i-string. See PEP 498 for some examples.
template string. See PEP 498 for some examples.
Different interpolation functions may also impose additional runtime
Different interpolators may also impose additional runtime
constraints on acceptable interpolated expressions and other formatting
details, which will be reported as runtime exceptions.
Leading whitespace in expressions is not skipped
------------------------------------------------
Unlike PEP 498, leading whitespace in expressions doesn't need to be skipped -
'$' is not a legal character in Python's syntax, so it can't appear inside
a ``${}`` field except as part of another string, whether interpolated or not.
Internationalising interpolated strings
=======================================
So far, this PEP has said nothing practical about internationalisation - only
formatting text using either str.format or bytes.__mod__ semantics depending
on whether or not a str or bytes object is being interpolated.
Since this PEP derives its interpolation syntax from the internationalisation
focused PEP 292, it's worth considering the potential implications this PEP
may have for the internationalisation use case.
Internationalisation enters the picture by overriding the ``__interpolate__``
builtin on a module-by-module basis. For example, the following implementation
would delegate interpolation calls to string.Template::
Internationalisation enters the picture by writing a custom interpolator that
performs internationalisation. For example, the following implementation
would delegate interpolation calls to ``string.Template``::
def _interpolation_fields_to_template(fields, expressions):
if not all(expr.isidentifier() for expr in expressions):
raise ValueError("Only variable substitions permitted for il8n")
template_parts = []
for literal_text, field_num, format_spec, conversion in fields:
if format_spec:
raise ValueError("Format specifiers not permitted for il8n")
if conversion:
raise ValueError("Conversion specifiers not permitted for il8n")
template_parts.append(literal_text)
if field_num is not None:
template_parts.append("${" + expressions[field_num] + "}")
return "".join(template_parts)
def __interpolate__(fields, expressions, values):
catalog_str = _interpolation_fields_to_template(fields, expressions)
translated = _(catalog_str)
values = {k:v for k, v in zip(expressions, values)}
@interpolator
def i18n(template, fields, values):
translated = gettext.gettext(template)
values = _build_interpolation_map(fields, values)
return string.Template(translated).safe_substitute(values)
If a module were to import that definition of __interpolate__ into the
module namespace, then:
def _build_interpolation_map(fields, values):
field_values = {}
for literal_text, field_num, expr, conversion, format_spec in fields:
assert expr.isidentifier() and not conversion and not format_spec
if field_num is not None:
field_values[expr] = values[field_num]
return field_values
* Any i"translated & interpolated" strings would be translated
* Any iu"untranslated & interpolated" strings would not be translated
* Any ib"untranslated & interpolated" strings would not be translated
* Any other string and bytes literals would not be translated unless explicitly
passed to the relevant translation machinery at runtime
And would then be invoked as::
This shifts the behaviour from the status quo, where translation support needs
to be added explicitly to each string requiring translation to one where
opting *in* to translation is done on a module by module basis, and
individual interpolated strings can then be opted *out* of translation by
adding the "u" prefix to the string literal in order to call
``__interpolateu__`` instead of ``__interpolate__``.
print(!i18n "This is a $translated $message")
Any actual implementation would need to address other issues (most notably
message catalog extraction), but this gives the general idea of what might be
possible.
It's also worth noting that one of the benefits of the ``$`` based substitution
syntax in this PEP is its compatibility with Mozilla's
`l20n syntax <http://l20n.org/>`__, which uses ``{{ name }}`` for global
substitution, and ``{{ $user }}`` for local context substitution.
With the syntax in this PEP, an l20n interpolator could be written as::
translated = !l20n "{{ $user }} is running {{ appname }}"
With the syntax proposed in PEP 498 (and neglecting the difficulty of doing
catalog lookups using PEP 498's semantics), the necessary brace escaping would
make the string look like this in order to interpolating the user variable
while preserving all of the expected braces::
interpolated = "{{{{ ${user} }}}} is running {{{{ appname }}}}"
Discussion
==========
@ -344,19 +378,42 @@ Discussion
Refer to PEP 498 for additional discussion, as several of the points there
also apply to this PEP.
Preserving the unmodified format string
---------------------------------------
Compatibility with IPython magic strings
----------------------------------------
A lot of the complexity in the il8n example is actually in recreating the
original format string from its component parts. It may make sense to preserve
and pass that entire string to the interpolation function, in addition to
the broken down field definitions.
IPython uses "!" to introduce custom interactive constructs. These are only
used at statement level, and could continue to be special cased in the
IPython runtime.
This approach would also allow translators to more consistently benefit from
the simplicity of the PEP 292 approach to string formatting (in the example
above, surrounding braces are added to the catalog strings even for cases that
don't need them)
This existing usage *did* help inspire the syntax proposed in this PEP.
Preserving the raw template string
----------------------------------
Earlier versions of this PEP failed to make the raw template string available
to interpolators. This greatly complicated the i18n example, as it needed to
reconstruct the original template to pass to the message catalog lookup.
Using a magic method rather than a global name lookup
-----------------------------------------------------
Earlier versions of this PEP used an ``__interpolate__`` builtin, rather than
a magic method on an explicitly named interpolator. Naming the interpolator
eliminated a lot of the complexity otherwise associated with shadowing the
builtin function in order to modify the semantics of interpolation.
Relative order of conversion and format specifier in parsed fields
------------------------------------------------------------------
The relative order of the conversion specifier and the format specifier in the
substitution field 5-tuple is defined to match the order they appear in the
format string, which is unfortunately the inverse of the way they appear in the
``string.Formatter.parse`` 4-tuple.
I consider this a design defect in ``string.Formatter.parse``, so I think it's
worth fixing it in for the customer interpolator API, since the tuple already
has other differences (like including both the field position number *and* the
text of the expression).
References
==========